1. Field of the Invention
The present invention relates to a sheet original feeding apparatus which feeds a sheet original to a predetermined position such as an image forming portion in an image forming system such as a copying machine, laser beam printer and the like, and an image forming system used with such sheet original feeding apparatus.
2. Related Background Art
In the past, one kind of sheet original feeding apparatus wherein an original is automatically fed to and rested on a predetermined position on a platen of a copying machine (as an image forming system) and the treated original is returned to an original stacking tray has been proposed. Such original feeding apparatus includes a circulating original feeding apparatus of switch-back type (referred to as "switch-back RDF" hereinafter) as shown in FIG. 1 and a circulating original feeding apparatus of closed-loop type (referred to as "closed-loop RDF" hereinafter) as shown in FIG. 2.
In the switch-back RDF shown in FIG. 1, an original is separated one by one from the bottom of an original stack P rested on an original tray 1 and is fed to a predetermined position on a platen 3 of a copying machine A. The original is stopped there, and an image on the original is read by an optical mechanism of the copying machine. After the image has been read, the sheet original P is conveyed from the platen 3 by switching back the sheet original P toward a same direction as a direction that the original is fed to the platen and is returned onto the original stack P in the original tray 1 by feeding the sheet original by means of a large roller 49 rotated in a direction shown by the arrow and a small roller 49a abutted against the large roller. Thus, the sheet original P is fed from and returned to the same side of the original tray, and, by repeating the above-mentioned sequences, the image forming operation regarding a plurality of originals P is effected.
In the switch-back RDF C of this kind, each original (the image thereon having been treated) is returned to the original stack P on the original tray from the side facing the leading ends of the originals in the stack, and a trailing end of the returned original P is aligned with the leading end of the original stack at the original supply side when the returned original slides down on the original stack toward the original supply side due to the inclination of the original tray. Therefore, even when the sheet originals P having different lengths in the original feeding direction are stacked on the original tray 1, the leading ends of the sheet originals are aligned with each other without fail.
Further, even regarding large-sized originals having different lengths in the original feeding direction, by extending the length of the original tray 1 by using an auxiliary tray, the sheet originals P can be re-stacked on the original tray 1 with good alignment of the leading ends thereof, without affecting a bad influence upon the original feeding path and/or the feeding operation.
Further, in the switch-back RDF C of this kind, in the case where the sheet original P rested on the platen 3 is replaced by a next sheet original P, when the leading end of the first original is positively pinched by the feeding means nearest the platen at a downstream side of the platen 3 (i.e., the large roller 49 and small roller 49a), a moving direction of a wide belt 2 which is a feeding means on the platen 3 is reversed to feed the next original to the platen, thus the first original (original after treated) and the next original (before treated) pass each other when exchanging the originals. In this way, the original exchanging time is shortened and the productivity of the original treatment is increased.
On the other hand, in the closed-loop RDF D shown in FIG. 2, an original is separated one by one from the bottom of an original stack P rested on an original tray 1 and is fed to a predetermined position on a platen 3 of a copying machine A and is stopped there. After the image of the original has been read, the original P is conveyed from the platen 3 toward a direction (right in FIG. 2) opposite to a direction that the original is fed to the platen and is returned onto the original stack P in the original tray 1 from a side opposite to an original supply side. By repeating this operation, the originals are circulated in a closed-loop path.
Further, since the feeding of the treated original can be initiated immediately after the image of the original has been treated and at the same time the feeding of the next original to the platen 3 can be started, the original exchanging operation can be performed without any time loss, thus further shortening the original exchanging time in comparison with the above-mentioned switch-back RDF C.
However, in the switch-back RDF C shown in FIG. 1, since there was the time loss between the completion of the treatment of the first original and the initiation of the feeding of the next original (that is to say, the feeding of the next original cannot be initiated before the first treated original is pinched by the downstream feeding means), the improvement in the productivity of the original treatment was limited to a certain extent
To the contrary, in the closed-loop RDF D shown in FIG. 2, there was less time loss in comparison with the switch-back RDF C. However, in the closed-loop RDF, the treated original is re-stacked on the original stack P from the side opposite to the original supply side of the original tray 1. Thus, if the originals P having different lengths in the original feeding direction are mixed in the original stack, since an original returning inlet is positioned behind a trailing end of the longest original in the original tray, it is feared that the leading end of the original to be ejected will strike against the trailing end of the original rested on the original tray to be fed, with the result that a stable and reliable original circulating operation cannot be attained. Accordingly, in such original feeding apparatus, only originals having the same lengths can be treated, and the originals having different lengths cannot be treated.
Further, in order to prevent the leading end of the ejected original from striking the trailing end of the original stack P rested on the original tray, noting that the original returning inlet must be positioned near the trailing end of the original stack P rested on the original tray 1, it is necessary to vary the position of the original returning inlet in accordance with the size of the sheet original P. Consequently, since the length of an original ejecting path extending from the platen 3 to the original returning inlet must also be varied, there arise disadvantages causing the construction of the original feeding apparatus itself to become complicated and the cost of the apparatus to increase. Further, since the construction for varying the length of the original ejecting path is considerably limited, the width of available originals is inevitably limited.